In a hydrokinetic torque converter transmission engine torque is distributed directly to the impeller of a hydrokinetic torque converter. The impeller and a turbine are disposed in a closed toroidal fluid flow circuit that includes a bladed stator located at the flow exit section of the turbine and the flow entrance section of the impeller. Engine torque thus is multiplied by the converter during operation in the converter torque multiplication mode. The multiplied torque of the turbine is distributed to the torque input elements of a multiple speed ratio planetary gear unit, the output of which is transferred to the vehicle traction wheels.
When the torque converter approaches a hydrokinetic coupling point as the vehicle is operating under a steady state cruising condition, a continuous slip occurs in the converter by reason of the hydrokinetic action of the turbine and the impeller. The converter thus contributes to smooth torque transfer from the engine to the traction wheels, but it is inherently inefficient because of the hydrokinetic losses in the converter.
It is known in the prior art to provide a lockup clutch to connect directly the impeller and the turbine during steady state cruising thereby eliminating the undesirable slippage which contributes to inefficiency. In a typical driveline the speed ratio that might occur under moderate highway speeds would be approximately 85% to 90%.
Examples of this can be seen by referring to U.S. Pat. No. 3,252,352 issued to Norman T. General, Po-lung Liang, and Robert P. Zundel on May 24, 1966. Another example is seen in U.S. Pat. No. 3,497,043 issued to Richard L. Leonard in February 1970. Both of these patents are assigned to the assignee of this invention.
Known bypass clutches for converters of this kind establish a fully mechanical torque transfer that bypasses the hydrokinetic unit. This introduces a source for undesirable transient torque fluctuations and tends to increase the noise, vibration and harshness of the driveline both during steady state operation and during transient operating conditions when the clutch is released or applied--for example, during shifts. One attempt to solve this problem of noise, vibration and harshness involves introducing in the driveline a damper that establishes a resilient connection between the output torque transfer element of the lockup clutch and the turbine shaft. Usually spring means are used in the damper in cooperation with friction coulomb devices for cushioning transient torque fluctuations while introducing a damper action that tends to stabilize the torque transfer.
Other prior teachings deal with lockup clutches and controls that will achieve a controlled slippage of the clutch during torque transmission so that the benefits of the hydrokinetic torque multiplication can be partially achieved while a portion of the driving torque is transmitted mechanically through the slipping bypass clutch. Such clutch designs are feasible in certain driveline arrangements in the automotive industry that employ friction materials that will be compatible with long term continuous slipping and that will make provision for dissipation of the heat that is generated because of the slipping action of the clutch. Examples of such a slipping clutch arrangement are shown in U.S. Pat. No. 4,468,988, issued Sep. 4, 1984, U.S. Pat. No. 4,660,697 issued April 28, 1987, and U.S. Pat. No. 4,725,951 issued Feb. 16, 1988. In the modulated bypass clutch of the '988 patent the circuit pressure in the converter torus is controlled electronically so that a desired degree of slippage occurs depending upon the driving conditions. The circuit pressure is used as the clutch actuating pressure that is applied to a clutch disc that cooperates with the friction surface carried by the impeller housing, the clutch disc in turn being connected resiliently to the torque converter turbine.
In controlling the magnitude of the clutch slippage during operation of the system of the '988 patent a target slip is set according to a manifold pressure or throttle position and engine speed. Sensors are used for detecting whether the engine, manifold pressure or throttle setting and the turbine shaft speed are related in such a way that a so called engagement zone for the clutch is effective. Both engine speed and manifold pressure or throttle position are used together with other engine variables to detect the operating condition of the engine.
In the '951 patent a torque converter lockup clutch control establishes a calculated converter slip range by controlling the duty cycle for a pulse width modulated solenoid control valve that in turn controls the pressure that actuates the clutch. The duty cycle is determined in accordance with the load on the engine so the lockup clutch is controlled in accordance with the gain for the solenoid valve whereby slippage in the torque converter is allowed to adjust to a preset value as the load changes.
The '697 patent describes a slipping bypass clutch control that calculates a time change in the slip during operation of the driveline and calculates a slip deviation between a preset target slip and the calculated slip. Engaging force of the clutch is controlled by a duty cycle controlled solenoid valve which acts on a regulator valve for the converter clutch. The duty cycle for the solenoid valve is adjusted in accordance with the computation of the controller which computes a corrected slip during each background control loop which takes into account the slip error that was determined in the previous background loop.
The controller of the '951 patent also uses a solenoid valve for controlling a regulator valve for the clutch. A control gain for the solenoid valve is altered in accordance with a computed value that is a function of the throttle opening and the engine speed. At any instant the computed value can be larger or smaller than a desired slippage, and either a positive compensation or a negative compensation occurs depending upon the output signal or gain that is calculated.
Another related prior teaching is found in Burcz et al U.S. Pat. No. 4,090,417 which shows a lockup torque converter control wherein a capacity modulator valve is used to establish a modulated pressure behind a clutch plate in order to reduce the engaging force in those instances when the engine is operating with partial throttle thereby inducing a slip and establishing a condition of incipient slip. This eliminates harshness in the engagement of the clutch. The capacity modulator valve functions to modulate the pressure made available behind the clutch plate in accordance with the engine throttle setting. Thus the capacity of the clutch is matched to the torque being distributed by the engine through the driveline under each operating condition.